1. Field of the Invention
The present invention relates generally to coatings comprising titanium and aluminum on substrates, methods for forming the coatings, and titanium and aluminum-containing materials as sputtering targets.
2. Description of Technical Considerations
Technology for depositing specific types of metallic or metal oxide-containing coatings on larger area substrates includes vapor deposition, such as chemical vapor deposition; spray pyrolysis; sol-gel; and sputtering, such as magnetic sputtering vapor deposition (“MSVD”). Larger area substrates of around 1 square foot (30 square centimeters) and larger provide challenges in economically consistent production of quality coated substrates by virtue of the size of the substrate that is coated. Consistency in coating uniformity and reduction of defects in the coating of larger areas require equipment that is able to handle the larger substrates, the volume of coating material, and the fabrication of the coated substrate. Such equipment is generally more expensive to purchase and operate; thus making efficient operation of the equipment imperative for cost-effective production.
Specific metal-containing (metals and/or metal oxides) coatings on substrates can exist as multi-layered coatings in which each layer is comprised of the same or different materials from one or more applications of the coating materials or precursors. Also, a layer of the coating can have one or more films from more than one application of the same or different materials. Examples of multi-layered coatings on a substrate are conventional silver-based low emissivity coatings that are deposited on both glass and plastic substrates, generally by sputtering.
In sputtering to deposit metals and metal oxides on larger surface area substrates, such as sheets or panels of light transmitting materials (e.g., plastic or glass), cathode targets have been used of the specific metal for deposition as the metal or metal oxide on the substrate. For larger area substrates of plastic and glass, such as or metal oxide on the substrate. For larger area substrates of plastic and glass, such as float glass with a surface area of at least 1 square foot (30 square centimeters), elongated cathode targets have been used. The targets are elongated to a length substantially the length or width of the substrate to be coated. For example, U.S. Pat. Nos. 4,990,234 and 5,170,291 to Szczyrbowski et al. and U.S. Pat. No. 5,417,827 to Finley disclose sputtering silica and silicides, such as transition metal silicide (NiSi2), in an oxidizing atmosphere to deposit dielectric oxide films.
U.S. Pat. No. 5,320,729 to Narizuka et al. discloses a sputtering target with which a high resistivity thin film consisting of silicon, titanium and aluminum, and oxygen can be produced. The target is formed by selecting the grain size of silicon powder and titanium and aluminum dioxide powder drying the powders by heating and mixing the dried powders to obtain a mixed powder containing from 20 to 80 percent by weight of silicon, for example 50 to 80 percent, the remainder being titanium and aluminum dioxide, packing the mixed powder in a die, and sintering the packed powder by hot pressing to produce a target which has a two-phase mixed structure. The sputtering target is used to manufacture thin film resistors and electrical circuits.
Sputtering cathode targets of various metallic materials are useful in vacuum deposited low emissivity (“Low-E”) coating stacks which usually have the following general layer sequence: S/(D1/M/P/D2)R where:                S is a substrate, such as a transparent substrate like glass;        D1 is a first transparent dielectric layer, usually a metal oxide, and can include one or more transparent dielectric films;        M is an infrared reflective layer, usually silver or other noble metal;        P is a primer layer to protect the underlying infrared reflective layer;        D2 is a second transparent dielectric film similar to D1; and        R is an integer equal to or greater than one and is the number of repetitions of the above layers.        
The dielectric layers, D1 and D2, adjust the optical properties of the coating stack. These layers also provide some physical and chemical protection to the fragile infrared reflective layer(s). Unfortunately, many process-friendly and cost-effective dielectric materials are often susceptible to abrasion and corrosion as well. For example, zinc oxide, e.g., as disclosed in U.S. Pat. No. 5,296,302, which usually forms a crystalline film, is susceptible to attack by acids and bases; bismuth oxide, which usually forms an amorphous film, is soluble in certain acids; tin oxide, which usually forms an amorphous film, is susceptible to attack in certain basic environments.
The P primer or blocker layers, as they are known in the art, are incorporated into such low emissivity coatings to protect the M layer or film from oxidation during the sputtering process. The M layer, like silver, is susceptible to breakdown during deposition of the overlying dielectric layer or film if the oxygen to reactive gas ratio is high, e.g., greater than 20 percent of the gas volume. The primer layers, which can be composed of pure metal layers or ceramic layers, act as sacrificial layers by preferentially oxidizing to protect the underlying silver layer or film. Generally thicker primer layers are necessary if the low emissivity coating is to survive the high temperature of a glass fabrication process (up to 650° C. or 1202° F.), e.g., bending and tempering of soda-lime glass.
To reduce corrosion, some Low-E coating stacks have an overlaying protective overcoat of a chemically-resistant dielectric layer. This layer has desirable optical properties, manageable sputter deposition characteristics, and is compatible with other materials of the coating stack. The titanium dioxide films disclosed in U.S. Pat. Nos. 4,716,086 and 4,786,563 are protective films having the above qualities. There are other chemically-resistant materials that have limitations, e.g., are more challenging to sputter. Silicon oxide disclosed in Canadian Patent No. 2,156,571, aluminum oxide and silicon nitride disclosed in U.S. Pat. Nos. 5,425,861; 5,344,718; 5,376,455; 5,584,902; and 5,532,180, and in PCT International Publication No. WO 95/29883 are examples of such materials. The sputtered multi-layered silver-based low emissivity coatings and glass with these coatings are used in automotive and window glazing applications.
It is known that the primer layer continues to oxidize during high temperature processing, and it is desirable for the oxidation to continue to completion in order to reduce visible light absorption from the primer layer. This effect is better utilized for metals that form metal oxides with low absorption coefficients, e.g., titanium and aluminum. For performance glazing applications, this leads to a higher visible light transmission to infrared transmittance ratio. If the oxidation continues beyond consumption of the primer layer to full oxidation, the coating can degrade and performance can suffer. Metal ions in the dielectric layers can inter-diffuse with the silver layer, and the well-defined interface can become fuzzy. This can lead to a loss of the antireflective behavior and loss of a continuous silver layer. The degree of oxidation of the primer is related to several factors, including the reactivity of the metal (Gibbs free energy), the density of the oxide formed during heating, and the diffusion or dissolution of oxygen in the oxide or metal. For example, a metal, such as titanium, in a thin film of less than around 20 Angstroms will pass through several oxidation states before reaching the thermally stable phase of TiO2. Titanium has been a preferred choice of material for primer layers in low emissivity multi-layered coatings.
The technology of metal and metallic coatings and multi-layered coatings would be advanced by a more chemically and/or mechanically durable coating that could be used as a protective coat for the substrate or multi-layered coated substrate or also useful as a dielectric or primer layer in multi-layered coatings on substrates. Additionally, it would be advantageous to provide methods for forming coatings having a more uniform concentration across the substrate to provide more uniform coating properties over the substrate. Moreover, it would be advantageous to provide a coating of differing concentration across the substrate if so desired to form areas of differing coating properties.